1. Field of the Invention
This invention relates to the suppression of undesirable phases in austenitic stainless steels. More particularly this invention relates to the suppression of chi and sigma phases in austenitic stainless steels containing a quantity of delta ferrite phase. As used herein, the terms "ferrite" and "delta ferrite" are synonymous. Steels containing delta ferrite are widely used as weld deposits for joining austenitic stainless steel components.
2. Description of the Prior Art
It has been known for many years that the crack sensitivity of austenitic stainless steel weld deposits could be reduced by the presence of delta ferrite in the deposit. It was recognized, however, that delta ferrite was undesirable in the weld deposit when the temperature and time cycles encountered during manufacturing or service permitted the transformation of delta ferrite to embrittling sigma phase. Consequently, for many high-temperature applications a maximum ferrite limit is imposed. For example, a 4-8% ferrite limit is imposed in weld deposits used for joining type 316 stainless steel in nuclear reactor components.
In the welding art it is customary to estimate the amount of ferrite present in a weld deposit based upon the composition of the filler metal, e.g., the consummable electrode in metal arc welding, and then to determine the amount of ferrite actually produced by measuring the magnetic properties of the weld. This ferrite estimate, sometimes called the ferrite number, is normally based upon the well-known Schaeffler constitution diagram (or a modification thereof) which correlates the amount of ferrite with the equivalent amounts of chrome and nickel in the alloy. Measurements of the ferrite content of the weld deposit are customarily made with an apparatus known as a Magna-Gage. The use of Schaeffler-type diagrams and Magna-Gages is more fully described by DeLong et al. in "Measurement and Calculation of Ferrite in Stainless Steel Weld Metal," Welding Research, Vol. 35, pp. 521s-528s (Nov. 1956), which is incorporated herein by reference.
It has been concluded by some workers in the prior art that chi phase as well as sigma phase participates in the embrittlement of austenitic stainless steels; see, for example Hull, F. C., "Effects of Composition on Embrittlement of Austenitic Stainless Steels," Welding Journal Research Supplement, Vol. 52, pp. 104-113 (March 1973). Hull pointed out that some alloys appear to be essentially immune to sigma and chi precipitation. It was observed that the tendency for embrittlement to occur upon aging generally increases as chromium is increased to improve oxidation or corrosion resistance; as molybdenum is increased to decrease pitting attack; as molybdenum, tungsten, or vanadium are added to increase strength; as titanium, columbium (niobium), or tantalum are added to stabilize carbon; or, as nickel is decreased to reduce cost or to provide delta ferrite.
Though much work has been done in an effort to explain sigma phase formation, it is still not fully understood. For example, Weiss, B. et al. in "Phase Instabilities During High-Temperature Exposure of 316 Austenitic Stainless Steel," Metalurgical Transactions, Vol. 3, pp. 851-866 (April, 1972), suggest that carbide precipitation causes the formation of sigma phase by reducing the carbon content in the matrix metal.
Stiegler, J. O. et al. in "Effect of Residual Elements on Fracture Characteristics and Creep Ductility of Type 308 Stainless Steel Weld Metal," Journal of Engineering Materials and Technology, pp. 245-250 (July 1975), suggest that control of the residual amounts of boron, phosphorus and titanium increases the ductility of the weld deposit due to the prevention of internal crack formation at austenite/sigma interfaces, possibly retarding the formation of sigma phase. Precipitation of carbon and other interstitials from austenitic stainless steels was said to soften the welds and permits more deformation before failure occurs. Carbon, phosphorus, boron, and sulfur added singly produced stronger weld metal than conventional welds. Boron and phosphorus additions produced welds with increased creep ductility as well. Sigma phase formed in all of the reported weld deposits.